1. Field of the Invention
The invention relates generally to fuel cell systems, and more particularly, the invention relates to methods and apparatuses for management of effluent products produced during an electrochemical reaction in a direct oxidation fuel cell system.
2. Background of the Invention
Fuel cells are devices in which an electrochemical reaction is used to generate electricity. A variety of materials may be suitable for use as a fuel depending upon the materials chosen for the components of the cell and the intended application for which the fuel cell will provide electric power.
Fuel cell systems may be divided into “reformer based” systems (which make up the majority of currently available fuel cells), in which fuel is processed to improve fuel cell system performance before it is introduced into the fuel cell, and “direct oxidation” systems in which the fuel is fed directly into the fuel cell without internal processing.
Because of their ability to provide sustained electrical energy, fuel cells have increasingly been considered as a power source for smaller devices including consumer electronics such as portable computers and mobile phones. Accordingly, designs for both reformer based and direct oxidation fuel cells have been investigated for use in portable electronic devices. Reformer based systems are not generally considered a viable power source for small devices due to size and technical complexity of present fuel reformers.
Thus, significant research has focused on designing direct oxidation fuel cell systems for small applications, and in particular, direct systems using carbonaceous fuels including methanol, butanol, propanol, and formaldehyde. One example of a direct oxidation fuel cell system is a direct methanol fuel cell system. A direct methanol fuel cell power system is advantageous for providing power for smaller applications since methanol has a high energy density (providing compact energy storage), can be stored and handled with relative ease, and because the reactions necessary to generate electricity occur under ambient conditions.
DMFC power systems are also particularly advantageous since they are environmentally friendly. The chemical reaction in a DMFC power system yields only carbon dioxide and water as by products (in addition to the electricity produced). Moreover, a constant supply of methanol and oxygen (preferably from ambient air) can continuously generate electrical energy to maintain a continuous, specific power output. Thus, portable computers, mobile phones and other portable devices can be powered for extended periods of time while substantially reducing and potentially eliminating at least some of the environmental hazards and costs associated with recycling and disposal of alkaline, Ni-MH and Li-Ion batteries.
The electrochemical reaction in a DMFC power system is a conversion of methanol and water to CO2 and water. More specifically, in a DMFC, methanol in an aqueous solution is introduced to an anode chamber side of a protonically-conductive, electronically non-conductive membrane in the presence of a catalyst. When the fuel contacts the catalyst, hydrogen atoms from the fuel are separated from the other components of the fuel molecule. Upon closing of a circuit connecting a flow field plate of the anode chamber to a flow field plate of the cathode chamber through an external electrical load, the protons and electrons from the hydrogen atoms are separated, resulting in the protons passing through the membrane electrolyte and the electrons traveling through an external load. The protons and electrons then combine in the cathode chamber with oxygen producing water. Within the anode chamber, the carbon component of the fuel is converted by combination with water into CO2, generating additional protons and electrons.
The specific electrochemical processes in a DMFC are:
Anode Reaction:CH3OH + H2O = CO2 + 6H+ + 6eCathode Reaction:O2 + 6H+ + 4e = 2H2ONet Reaction:CH3OH + 3/2O2 = CO2 + H2O
The methanol in a DMFC is preferably used in an aqueous solution to reduce the effect of “methanol crossover”. Methanol crossover is a phenomenon whereby methanol molecules pass from the anode side of the membrane electrolyte, through the membrane electrolyte, to the cathode side without generating electricity. Heat is also generated when the “crossed over” methanol is oxidized in the cathode chamber. Methanol crossover occurs because present membrane electrolytes are permeable (to some degree) to methanol and water.
One of the problems with using DMFC power systems in portable power applications is the lack of a low-cost, effective method and system for removing effluents produced by the electrochemical reaction generally, and in particular, to remove water generated on the cathodic face of the membrane electrolyte or otherwise present in the cathode chamber. If water generated in the cathode chamber collects on the cathode of the membrane or in the anode chamber, it may prevent oxygen from coming into contact with the cathodic electrocatalyst, interrupting productive oxidation of the fuel and generation of electricity.
In addition, the proper ratio of fuel to water delivered to the anode chamber in DMFC power systems must be maintained. During operation, water molecules may be pulled across the membrane with hydrogen protons leading to excess water on the cathode side of the membrane and an increase in methanol concentration at the anode. The increased concentration of methanol may lead to additional methanol crossover resulting in decreased efficiency, a waste of methanol, and the generation of unwanted heat.
Theoretically, the effluents could be removed by venting the carbon dioxide out of the anode chamber and evaporating the water from the cathode side of the membrane electrolyte with a low humidity ambient airflow. However, under many relevant conditions (e.g., low volume air flow, low ambient air pressure, moderate to high humidity), the water cannot be effectively removed, and thus, alternate methods of eliminating water generated in the cathode are required.
According, the suitability of DMFC power systems for powering portable devices and consumer electronics is dependent upon the development of systems and methods for eliminating and/or recirculating the effluent products produced during operation of the fuel cell. In addition, in order for DMFC power systems to be used effectively, they must be self-regulating and passively generate electrical power under benign operating conditions, such as ambient air temperature and pressure.